The present invention pertains to the art of electromagnetic geophysical surveys conducted near a borehole and, more specifically, to mitigating the effect of near surface geological structures on electromagnetic data collected during the surveys. The embodiments described herein relate generally to soundings within the Earth based upon electrical or magnetic fields. As used herein, “Earth” generally refers to any region in which a borehole may be located including, for example, the lithosphere. Electromagnetic geophysical surveys probe electrical resistivity, or equivalently, conductivity, in the ground as a function of depth. While the term “electromagnetic” is used generally, the term is intended to cover electric and/or magnetic or even induced polarization techniques.
The source of the electromagnetic field used in a geophysical survey may originate in the natural environment or be manmade. Generally known methods employ transmitters that induce electrical currents to flow in the ground. The transmitters are preferably sources of electric current injected by electrodes implanted in the soil or rock and connected to a power supply or the transmitters are loops of wire carrying an alternating current which produces an alternating magnetic field that, by Faraday's law of induction, induces an electromotive force in the ground that, in turn, drives currents in the ground. In either case, the currents induced depend on the distribution of resistivity in the ground and these induced currents produce secondary electric and magnetic fields that are measured by receivers which are usually separated from the transmitter. For instance, the receivers may include two separated electrodes in contact with the ground and across which a voltage is measured that is proportional to the electric field at that point. Receivers may also include a variety of sensors designed to measure the magnetic fields that accompany the induced currents. The transmitters and receivers can be on the surface or in the ground.
These methods are used to determine the distribution of electrical resistivity in the ground. For example, the methods are preferably used to characterize the layering of the ground so as to identify an electrically resistive (high resistivity) layer that contains oil or gas, an electrically conductive (low resistivity) layer containing saline water, or a clay layer that might be an impermeable barrier for hot water in a geothermal setting, or other targets of contrasting electrical resistivity with the background. A more specific application of such methods is to determine the size and electrical resistivity of limited regions in the ground. Examples are zones of petroleum rich rock in an oilfield that has not been drained by the existing oil wells in the field (essentially bypassed oil), zones of electrically conducting rocks reflecting the presence of metallic ore minerals, a zone of enhanced electrical conductivity brought about by the injection under pressure of a fluid mixture designed to cause a fracture or a fluid mixed with solid conductive particles intended to keep the fracture open (proppant), or a zone of changing electrical resistivity caused by the injection of carbon dioxide for sequestration, or for mapping or monitoring carbon dioxide, steam or chemical for enhanced oil recovery (tertiary recovery) or water for improved oil recovery (secondary recovery) or mapping and monitoring steam or chemicals injected to reduce viscosity and increase production from an oilfield formation. The application need not be restricted to exploring oilfields; other applications include pollutant remediation and groundwater exploration. In all of these applications, the goal is to detect and, if possible, delineate a zone whose electrical resistivity is distinctly different from the resistivity of the overall volume of the ground below the surface in a specified region (referred to as the background resistivity). Since the resistivities of such targets and the surrounding medium may be quite dissimilar, it is possible to discriminate between them by measuring their subsurface resistivities when subjected to an electromagnetic field. Using this methodology, the depth, thickness, and lateral extent of materials of interest can be determined. Combined with other data, volumes and saturation can be determined as well.
Most of the prior work in this area concentrated on using a surface to borehole configuration rather than a borehole to surface configuration. The methods are similar, differing only in whether the transmitter is located in the borehole or at the surface. An advance in EM methods specifically for the deep subsurface is described in International Patent Application No. PCT/US2012/39010, entitled “System and Method to Measure or Generate an Electrical Field Downhole” now published as U.S. Patent Application Publication No. 2015/0160364, by Hibbs and Glezer, which is incorporated herein by reference. A further advancement described in International Patent Application No. PCT/US2013/058158, entitled “System and Method to Induce an Electromagnetic Field within the Earth”, now published as U.S. Patent Application Publication No. 2015/0219784, by Hibbs and Morrison, which is also hereby incorporated by reference, is to remove the source electrode at depth within the casing and instead drive the entire casing of the borehole at the desired voltage, V, by making an electrical connection at or near the top of the casing.
Borehole to surface electromagnetics is emerging as a significant new method for imaging the Earth, especially in oil and gas fields and can produce accurate images of fluid distribution up to 2 km from a well. With this method, an electromagnetic (electric or magnetic) source is placed at depth, usually within a borehole, and when activated generates fields and currents within the subsurface that interact with subsurface structures. The total field is the sum of the provided signal (primary field) and that produced by this interaction (secondary field). The overall field is measured at the surface with an array of magnetic or electrical field detectors and the fields are inverted to yield an electrical resistivity distribution (or by inverse, conductivity distribution) that may be associated with targets of interest.
Borehole to surface electromagnetics is much more sensitive to subsurface structures than surface techniques due to the closer proximity of the source to the region of interest. That is, when the source is deployed close to the target region, the resulting field is much more sensitive to this body than a measurement with a more remote source. Unfortunately, although there is more sensitivity to structures near the transmitter antennas, there is also high sensitivity with structures near the receiver antennas, or the near surface geology, topography and infrastructure. That is, in areas of complex near surface or shallow geology, it may still be difficult to resolve deep targets from the influence due to “geologic noise”, which is defined as small scale undefined near surface geologic structures.
With the above in mind, there is seen to be a need for a system and method for removing near surface effects from borehole to surface electromagnetic data.